Extracting key frame candidates from video clip

ABSTRACT

A method of analyzing a digital video clip to determine candidate frames for subsequent key frame selection including performing a global motion estimate on the video clip that indicates translation of the scene or camera, or scaling of the scene; forming a plurality of video segments based on the global motion estimate and labeling each segment in accordance with a predetermined series of camera motion classes; extracting key frame candidates from the labeled segments and computing a confidence score for each candidate by using rules corresponding to each camera motion class and a rule corresponding to object motion.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is related to U.S. patent application Ser. No.______, filed ______ entitled “ANALYZING CAMERA CAPTURED VIDEO FOR KEYFRAMES” by Jiebo Luo, Christophe Papin, Deniz Schildkraut and MajidRabbani.

FIELD OF THE INVENTION

The invention relates generally to the field of digital image processingand, more particularly, to a method usable in extracting a key framefrom a video clip.

BACKGROUND OF THE INVENTION

Extracting key frames (KF) from video is of great interest in manyapplication areas. Main usage scenarios include printing from video(select or suggest the best frames to be printed), video summary (e.g.watch a wedding movie in seconds), video compression (optimize keyframes quality when encoding), video indexing, video retrieval, andvideo organization. In general, key frames should show good quality andhigh semantic interest. However, what exactly is a key frame sometimesdepends on the application. The level of requirement can also bedifferent. For printing still pictures from video, one needs to put astrong focus on image quality. For rapid browsing one will need toincrease the representativeness in semantics. Key frame extraction canbe a feature offered in a camera (including a digital camera, camcorder,and camera phone), in desktop image/video editing/management software,and with online image/video service.

Key frame extraction is not a new problem. However, prior art has beenfocused on sports or news video with constrained structures. Such videoconforms to well-defined common structures and characteristics. Forinstance, in field sports (including soccer, football, baseball, rugby,and cricket), there are two opposing teams and referees in distinctcolorful uniforms, an enclosed playing area on grass or artificial turf,field lines and goals, commentator voice and spectator cheering, andfinally, on-screen graphics (scoreboard). There are often a small numberof canonic “views”: field view, zoom-in, and close-up. Other types ofsports, such as racquet sports, basketball, as well as news videos,share a different set of structured characteristics. More importantly,there is unambiguous ground truth as to which are the key frames withinthe given context. In contrast, even the themed consumer videos (e.g.,wedding, birthday party) do not have the same level of common structuresand characteristics, and the key frame selection is open to a high levelof subjectivity because of observer association, sentimental values, andother factors.

In addition, image quality (contrast, exposure, camera shake) is rarelya concern for sports and news video because of superior imagingequipment and well-controlled imaging conditions. Example systems forextracting key frames from sports and news videos include Avrithis, Y.S., Doulamis, A. D., Doulamis, N. D., and Kollias, S. D., “A StochasticFramework for Optimal Key Frame Extraction from MPEG Video Databases,”Computer Vision and Image Understanding, 75(1/2), 1999, pp. 3-24; Liu,T., Zhang, H. J., and Qi, F., “A novel video key-frame-extractionalgorithm based on perceived motion energy model,” IEEE Trans. Cir. Sys.Video Techno, 13(10), 2003, pp. 1006-1013; Y. Rui, A. Gupta, and A.Acero, “Automatically extracting highlights for TV Baseball programs,”ACM Multimedia 2000, pp 105-115; B. Li and M. I. Sezan, “Event Detectionand Summarization in Sports Video,” IEEE Workshop on Content-basedAccess of Image and Video Libraries (CBAIVL), 2001, pp. 132-140.

Short movie clips captured by a digital camera with video capabilities(a recent product feature) are different. The variety in occasions andsituations for consumer videos is unconstrained. Contrary toprofessional videos, there are no special effects, no tightlypre-defined structure, no professional editing, and a video cliprepresents only one shot. In that sense, video summary from a short clipis potentially easier than for those recorded by a camcorder because onedoes not need to perform video shot segmentation. Camera shake is oftenpresent and exposure is often problematic compared to professionalvideos. Above all, the biggest challenge with consumer video is itsunconstrained content and lack of structure. Tong Zhang, in US patentapplication publication US 2005/0228849, “intelligent key-frame exactionfrom a video”, described a method for intelligent key frame extractionfor consumer video printing based on a collage of features includingaccumulative color histogram, color layout differences, camera motionestimation, moving object tracking, face detection and audio eventdetection. Specifically, Zhang disclosed a method for extracting a setof key-frames from a video, comprising the steps of: selecting a set ofcandidate key-frames from among a series of video frames in the video byperforming a set of analyses on each video frame, each analysis selectedto detect a meaningful content in the video; arranging the candidatekey-frames into a set of clusters; selecting one of the candidatekey-frames from each cluster in response to a relative importance ofeach candidate key-frame.

Because the application of key frame extraction can vary significantly,for example, in terms of the desired number of key frames, it is oftendesirable to implement a flexible framework capable of producing ascalable video representation. The optimal number of relevant key framesis highly dependent on the video complexity. Complexity is a function ofmany features: camera motion, scene content, action and interactionbetween moving objects, image quality (IQ) due to lightning and camerasetting, and so on. The video duration is also a parameter that coulddrive the video complexity: a longer movie clip is likely to containmore events and therefore demands more key frames.

One also need to define the best criteria of representativeness, andthen determine what features can be used to obtain the ‘best’ key framesgiven the input data. Different features, such as those used in US2005/0228849, vary significantly in terms of their effectiveness andcomputational cost. It is desirable to use as fewer features as possibleto achieve a reasonable performance with reasonable speed.

Furthermore, because video clips taken by consumers are unstructured,one should rely only on cues related to the cameraman's general intents,i.e., camera and object motion descriptors. Rules applicable only tospecific content only have limited use and need advance informationabout the video content.

Consequently, it would be desirable to design a system that is reliable,efficient, regardless of the image content.

SUMMARY OF THE INVENTION

The present invention is directed to overcoming one or more of theproblems set forth above. A method according to the present inventionanalyzes a digital video clip to determine candidate frames forsubsequent key frame selection, by:

a. providing a camera motion sensor in the camera so that information isprovided during image capture regarding camera motion includingtranslation of the scene or camera, or scaling of the scene;

b. forming a plurality of video segments based on the global motionestimate and labeling each segment in accordance with a predeterminedseries of camera motion classes; and

c. extracting key frame candidates from the labeled segments andcomputing a confidence score for each candidate by using rulescorresponding to each camera motion class and a rule corresponding toobject motion.

One aspect of the present invention, focuses on motion-based descriptorsas the most effective and efficient features. Camera and object motionsare estimated and used to derive a set of motion descriptors. The videoclip is divided into a set of homogeneous segments based on the majortypes of camera motion (pan, zoom . . . ). Dedicated rules allowcandidate key frames to be extracted from each segment. The use ofcomplementary descriptors based on image quality (IQ) or semanticanalysis (e.g., skin, face or expression) would enable more satisfactoryresults at an additional expense.

An important feature of the invention is ranking candidates. Aconfidence score is attached to each candidate and is later used to rankall candidates in an order of relevance. This leads to a scalableframework such that we are able to obtain an arbitrary subset of keyframes from candidates given the need of a particular application.

These and other aspects, objects, features and advantages of the presentinvention will be more clearly understood and appreciated from a reviewof the following detailed description of the preferred embodiments andappended claims, and by reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating an overview of the key frameextraction method according to the present invention;

FIG. 2 shows an illustration of a video clip containing several cameramotion classes and object motion classes, along with desired key frameextraction in response to such motion, in accordance with theinterpolation detection method shown in FIG. 1;

FIG. 3 shows a summary of the rules for key frame extraction in responseto the camera motion classification of the present invention;

FIG. 4 shows an illustration of a video clip for candidate extractionfrom a pan segment;

FIG. 5 shows an illustration of a video clip for candidate extractionfrom a pan segment containing pauses in camera motion; and

FIG. 6 shows an illustration of a video clip for candidate extractionfrom a zoom-in segment.

DETAILED DESCRIPTION OF THE INVENTION

Because many basic image and video processing algorithms and methods arewell known, the present description will be directed in particular toalgorithm and method steps forming part of, or cooperating more directlywith, the method in accordance with the present invention. Other partsof such algorithms and methods, and hardware or software for producingand otherwise processing the video signals, not specifically shown,suggested or described herein can be selected from such materials,components and elements known in the art. In the following description,the present invention will be described as a method typicallyimplemented as a software program. Those skilled in the art will readilyrecognize that the equivalent of such software can also be constructedin hardware. Given the system as described according to the invention inthe following materials, software not specifically shown, suggested ordescribed herein that is useful for implementation of the invention isconventional and within the ordinary skill in such arts.

It is instructive to note that the present invention utilizes a digitalvideo which is typically either a temporal sequence of frames, each ofwhich is a two-dimensional array of red, green, and blue pixel values oran array of monochromatic values corresponding to light intensities.However, pixel values can be stored in component forms other than red,green, blue, can be compressed or uncompressed, and can also includeother sensory data such as infrared. As used herein, the term digitalimage or frame refers to the whole two-dimensional array, or any portionthereof that is to be processed. In addition, the preferred embodimentis described with reference to a typical video of 30 frames per second,and a typical frame resolution of 480 rows and 680 columns of pixels,although those skilled in the art will recognize that digital videos ofdifferent frame rates and resolutions can be used with equal, or atleast acceptable, success. With regard to matters of nomenclature, thevalue of a pixel of a frame located at coordinates (x,y), referring tothe x^(th) row and the y^(th) column of the digital image, shall hereincomprise a triad of values [r(x,y), g(x,y), b(x,y)] respectivelyreferring to the values of the red, green and blue digital imagechannels at location (x,y). In addition, a frame is identified with atime instance t.

Referring to FIG. 1, there is shown an overview block diagram of thepresent invention. An input video clip 10 first undergoes global motionestimation 20. Based on the estimated global motion, the video clip 10is then divided through video segmentation 30 into a plurality ofsegments (which may or may not overlap), each segment 31 correspondingto one of a pre-determined set of camera motion classes 32, includingpan (left or right), tilt (up or down), zoom-in, zoom-out, fast pan, andfixed (steady). For each segment 31, key frame candidate extraction 40is performed according to a set of pre-determined rules 41 to generate aplurality of candidate key frames 42. For each candidate frame, aconfidence score (not shown) is also computed to rank all the candidates42 in an order of relevance. Final key frame selection 50 is performedaccording to a user-specified total number 51 and the rank ordering ofthe candidates. In a preferred embodiment of the present invention, thefinal key frames 52 include at least the highest ranked frame in eachsegment 31.

Because video clips taken by consumers are unstructured, rulesapplicable only to specific content only have limited use and also needadvance information about the video content for them to be useful. Ingeneral, one can only rely on cues related to the cameraman's generalintents. Camera motion, which usually corresponds to the dominant globalmotion, allows a prediction of the cameraman's intent. A “zoom in”indicates that he has an interest in a specific area or object. A camera“pan” indicates tracking a moving object or scanning an environment.Finally, a rapid pan can be interpreted as a lack of interest or a quicktransition toward a new region of interest (ROI). The secondary or localmotion is often an indication of object movements. These two levels ofmotion description combine to provide a powerful way for video analysis.

In a preferred embodiment of the present invention, the algorithm byJ.-M. Odobez and P. Bouthemy, “Robust Multiresolution Estimation ofParametric Motion Models,” J. Vis. Comm. Image Rep., 6(4):348-365, 1995,is used in global motion estimation 20 as a proxy for the camera motion.The method is summarized here. Let θ denote the motion-based descriptionvector. Its first 3 components correspond to the camera motion and arededuced from the estimation of a 6-parameter affined model that canaccount for zooming and rotation, along with simple translation. Thisdescriptor relies on the translation parameters a₁ and a_(2,) and theglobal divergence (scaling) div. The last descriptor evaluates theamount and the distribution of secondary motion. We refer to secondarymotion as the remaining displacement not accounted for by the globalmotion model. Such spatio-temporal changes are mainly due to objectsmoving within the 3D scene. The Displaced Frame Difference (DFD)corresponds to the residual motion once the camera motion iscompensated. We also combine spatial information (the average distanceof the secondary motion to the image center) and the area percentage ofthe secondary motion. The fourth component of θ is given by:$\begin{matrix}{{obj} = {{\omega_{dtc} \cdot \frac{1}{N_{\Lambda}}}{\sum\limits_{p \in \Lambda}{{th}_{Hyst}\left\lbrack {{{DFD}(p)}} \right\rbrack}}}} & (1)\end{matrix}$

The function th_(Hyst) relies on a hysteresis threshold, N_(Λ)is thenumber of active pixels p, and the normalized linear function ω_(dtc)favors centrally located moving areas.

A video can be characterized in terms of camera motion and objectmotion. Camera motion is fairly continuous and provides a meaningfulpartition of a video clip into homogeneous segments in step 30 ofFIG. 1. Object activity is an unstable but still useful feature.Referring to FIG. 2, this example video clip consists of the followingsequence of camera motion: pan (environment), zoom-in, zoom-out, fastpan, fixed, pan (tracking object), and fixed. Note that a “zoom in” canbe caused by a mechanical/optical action from the camera, by the motionof the cameraman (towards the object), or by the movement of the object(towards the camera). However, they are equivalent from an algorithmprospective as “apparent” zoom-in.

As for object motion, the example video clip in FIG. 2 consists of thefollowing sequence of object motion: no object motion, high objectmotion, and finally low object motion. Note that the boundaries of theobject motion segments do not necessarily coincide with the boundariesof the camera motion.

Continuing the reference to FIG. 2, according to the present invention,rules are formulated and confidence functions are defined to selectcandidate frames for each segment in step 40 of FIG. 1. For the firstsegment, which is a pan, it would be desirable to select two key framesto span the environment (as marked). For the subsequent zoom-in andzoom-out segments, a key frame should be selected at the end of eachsegment when the zooming action stops. It is usually not necessary toextract a key frame for the fast pan segment because it is merelytransition without any attention paid. Although object motion startsduring the latter stage of the fast pan, it is only necessary to extractkey frames once the camera becomes steady. One key frame should beextracted as the camera pans to follow the moving object. Finally, asthe object moves away from the steady camera, another key frame isselected.

The rules used in the above example are of a general purpose in nature.They do not rely on any semantic information on what the object is, whatthe environment is, or what the object motion is. Therefore, they can beapplied to any other video clips. These generic rules are summarized inFIG. 3.

The present invention distinguishes four camera motion-based classes:“pan,” “zoom in,” “zoom out,” and “fixed.” Note that “tilt” is handledin the same way as “pan” and is treated as the same class (withoutstraightforward modification). Also note that the descriptor obj is notused during video segmentation, which involves applying adaptivethresholds to the scaling and translation curves over time (per the6-parameter model). In the following, detailed descriptions are providedfor each camera motion class.

A slow camera pan takes more time to scan a significant area. It seemsappropriate to make the segmentation threshold depend on the pansegment's length l, but it is a chicken-and-egg problem because oneneeds to segment the translation data first to know the length itself.To overcome this problem, a small translation threshold value is used toprovide a rough segmentation. There would be no need to extract a pansegment if the camera view does not change significantly. The adaptivethreshold th_(pan) is lower when dealing with longer pan. In a preferredembodiment of the present invention, th_(pan) is defined as the unitamount of camera translation required to scan a distance equal to theframe width w multiplied by a normalized coefficient γ that represents avalue beyond which the image content is considered to be differentenough.

There exists a strong redundancy over time. To save computing time, itis advantageous not to estimate motion for every frame. Instead, aconstant temporal sampling rate is maintained over time regardless ofthe capture frame rate. Let t_(s) denote the temporal subsampling step(the capture frame rate divided by a fixed number of frame samples persecond). The time reference attached to the video is denoted as

₀ and represents the physical time. The second time reference, denoted

₁, is related to the subsampled time. Thus,l′·t _(s) ·th _(pan) =γ·w  (2)

The number of frames N is equal to l′·t_(s), where the duration l′ isconsidered in

1. Finally, the adaptive threshold is $\begin{matrix}{{th}_{pan} = \frac{\gamma \cdot w}{l^{\prime} \cdot t_{s}}} & (3)\end{matrix}$

A similar method is used to segment the scaling curve. In this case,there is no need to consider a minimal distance to cover but instead aminimum zoom factor. If the scaling process is short, its amplitude mustbe high enough to be considered. In reference

1, the scaling factor is generalized to $\begin{matrix}{f_{zoom} = {\prod\limits_{t \in l^{\prime}}\left\lbrack {1 + {{div}(t)}} \right\rbrack^{t_{s}}}} & (4)\end{matrix}$

If div(t) is assumed to be the threshold th_(zoom) and constant overtime, this expression can be compared to a desired total scaling factorγ_(s), reflecting the entire zoom motion along a given segment of lengthl′:([1+th _(zoom)]^(t) ^(s) )^(l′)=γ_(s)  (5)

Therefore, the adaptive zoom threshold is given by $\begin{matrix}{{th}_{zoom} = {{\exp\left( \frac{\ln\quad\gamma_{s}}{l^{\prime}t_{s}} \right)} - 1}} & (6)\end{matrix}$

The KF candidates form a fairly large set of extracted frames, each ofwhich is characterized by a confidence value. Although such a valuediffers from camera motion class to class, it is always a function ofthe descriptor's robustness, the segment's length, the motiondescriptor's magnitude, and the assumptions on the cameraman's intent.

In the present invention, high-level strategies are used to selectcandidates. They are primarily based on domain knowledge. A zoom-incamera operation generally focuses on a ROI. It can be caused by amechanical/optical action from the camera, movement of the cameraman, ormovement of the object. These scenarios are equivalent from thealgorithm's perspective as apparent zoom-in. It is desirable to focus onthe end of the motion when the object is closest.

Typically, a camera pan is used to capture the environment. Trackingmoving objects can also cause camera translations similar to a pan. Oneway to differentiate between the two scenarios is to make use of theobject motion descriptor obj. However, its reliability depends on theability to compensate for the camera motion. KF candidates are extractedbased on the local motion descriptor and the global translationparameters. Camera motion-dependent candidates are obtained according toa confidence function dependent on local translation at a minimal andcumulative panning distance. Other candidates are frames with largeobject motion.

Finally, for a “fixed” or steady segment, in one embodiment of thepresent invention, it is reasonable to simply choose the frame locatedat the midpoint of the segment. Preferred embodiments should useinformation from additional cues, including image quality (e.g.,sharpness, contrast) or semantic descriptors (e.g. facial expression) toselect the appropriate frame.

In a preferred embodiment of the present invention, the main goal is tospan the captured environment by a minimum number of KF. Because scenecontent in a consumer video is rarely static, one also needs to considerlarge object motion. Covering the spatial extent and capturing objectmotion activity are quite different in nature, and it is nontrivial tochoose a trade-off between them. Certainly, a lack of object motionsignifies that the cameraman's intent was to scan the environment. Inaddition, a higher confidence score is assigned to candidates based onthe cumulative distance.

To reduce spatial overlap, a probability function d_(spat) is formulatedas a function of the cumulative camera displacements. It is null at thesegment's onset and increases as a function of the cumulativedisplacements. The scene content is judged different enough whend_(spat) reaches 1. Once d_(spat) reaches 1, its value is reset to 0before a new process starts again to compute the cumulative cameradisplacements. To avoid a sharp transition, its value decreases rapidlyaccording to a Gaussian law to 0 (for instance within the next 3frames). Note that the cumulative camera displacement is approximatedbecause the camera motion is computed only every t_(s) frames. FIG. 4shows top candidate frames extracted by using only d_(spat). Each framecontains distinct content, i.e., to miss any one of them would be tomiss part of the whole landscape.

It is worthwhile considering the cameraman's subtler actions. It isnoticed that a pause or slow-down in pan often indicates a particularinterest, as shown in FIG. 4. It makes sense to assign higher importanceto such areas that are local translation minima using the probabilityfunction d_(know)=G(μ,σ), where the function G is a Gaussian function,with μ as the location of local minimum and u the standard deviationcomputed from the translation curve obtained upon global motionestimation. Example candidate frames extracted from function d_(know)are shown in FIG. 4. Because the candidate frames obtained from d_(spat)and d_(know) can be redundant, one needs to combine d_(spat) andd_(know) using a global confidence function d_(pan):d _(pan) =a ₁ d _(spat) +a ₂ d _(know)  (7)with a₁+a₂=1, such that d_(pan) lies between 0 and 1. Typically, onedoes not favor either criterion by selecting a₁=a₂=0.5.

Referring to FIG. 4, candidates are extracted from a pan segment wherethe pan speed is not constant (as indicated by the ups and downs in thecamera translation curve in the middle row). In the top row, six framesare extracted to span the environment while reducing their spatialoverlap. In the bottom row, additional five frames are selectedaccording to the minimum points in the translation curve.

Referring now to FIG. 5, there is shown an example of the functiond_(pan), with candidates extracted from a pan segment. Confidence valuesd_(pan) are used to rank candidate frames. Modes between 0 and 0.5 onlydisplay a high percentage of new content, while modes with valuesgreater than 0.5 correspond to a high percentage of new content and arealso close to a translation minimum (pan pause). Function d_(pan)enables us to rank such candidate frames.

Fast pan represents either a transition toward a ROI or the tracking ofan object in fast motion. In both cases, frames contain severe motionblur and therefore are not useful. It makes sense not to extract KF fromsuch segments. A normalized confident coefficient c based on thetranslation values is introduced. In a preferred embodiment of thepresent invention, the coefficient c is reshaped by a sigmoid function:$\begin{matrix}{{c(\omega)} = {\frac{1}{1 + {\mathbb{e}}^{4{k{({\omega - {th}_{High}})}}}}.}} & (8)\end{matrix}$where k is the slope at the translation threshold th_(high), andc(th_(high))=0.5. The coefficient c acts as a weighting factor ford_(pan):d _(pan) =c(ω)└a ₁ d _(spat) +a ₂ d _(know)┘  (9)

The coefficient c is close to 1 for small translation, decreases aroundth_(high) according to the parameter k, and eventually approaches 0 forlarge translations.

Candidate selection from a zoom segment is driven by domain knowledge,i.e., KF should be at the end of a zoom segment. The confidence functiond_(zoom) can be affected by translation because large pan motion oftencauses false scaling factor estimates. Similarly to Eq. 8, let c_(pan)denote a sigmoid function that features an exponential term based on thedifference between the Euclidian norm of the translation componentω₀(t), t being the time associated with the maximal zoom lying withinthe same segment of the candidate key frame, and a translation parametertr_(Max) (which can be different from th_(high)).

The coefficient c_(pan) provides a measure of the decrease in theconfidence of the scaling factor when large pan occurs. A high zoombetween two consecutive frames is unlikely due to the physical limits ofthe camera motor. Even though an object might move quickly toward thecamera, this would result in motion blur. In a preferred embodiment ofthe present invention, the maximal permitted scaling factor th_(s)between two adjacent frames is set to 0.1 (10%), and the f_(zoom) factorintroduced in Eq. 4 is modified to: $\begin{matrix}{f_{zoom} = {\prod\limits_{t \in l^{\prime}}{{\Xi\left( {{1 + {{div}(t)}},{th}_{s}} \right)}\left\lbrack {1 + {{div}(t)}} \right\rbrack}^{t_{s} - 1}}} & (10)\end{matrix}$where the step function is Ξ(x, a)={_(x if x<a) ^(0 if x≧a).

Finally, after applying normalization function N, Eq. 10 can berewritten as $\begin{matrix}{f_{zoom} = {\prod\limits_{t \in l_{k}^{\prime}}{N\left( {{\Xi\left\lbrack {1 + {{div}(t)}} \right\rbrack}\left\lbrack {1 + {{div}(t)}} \right\rbrack}^{t_{s}} \right)}}} & (11)\end{matrix}$and the confidence function d_(zoom) for a zoom candidate isd _(zoom) =c _(pan) ·f _(zoom)  (12)

Referring now to FIG. 6, there is shown an example of candidateextraction from a series of zoom-in segments. The top row is the plotfor (apparent) camera scaling. The bottom row displays the candidateframes rank ordered according to the confidence function d_(zoom). Theactual locations of these candidates are marked in the scaling curve.

Zoom-out segment is processed in a similar fashion, where candidates areextracted at the end of the segment. However, even though a zoom-outoperation could be of interest because it captures a wider view of theenvironment, extracting a candidate key frame from a zoom-out segment isoften redundant. The subsequent segment generally contains frames withsimilar content. In the present invention, a single candidate frame isextracted at the end of a zoom-out segment, but it will be compared tothe key frame(s) extracted in the next segment to remove any redundancy.To confirm any redundancy, the simplest metrics are histogram differenceand frame difference. In a preferred embodiment of the presentinvention, each frame is partitioned into the same number L of blocks ofsize M×N, color moments (mean and standard deviation) are computed foreach block. The corresponding blocks are compared in terms of theircolor moments. Two blocks are deemed similar if the distance between thecolor moments is below a pre-determined threshold. Two frames are deemedsimilar if the majority (e.g., 90%) of the blocks are similar.

Candidates are also selected based on object motion activity, which canbe inferred from the remaining displacement (secondary motion) that isnot accounted for by the global motion model. Such spatio-temporalchanges are mainly due to objects moving within the 3D scene. Largeobject motion is often interesting. Therefore, local maximum values ofthe descriptor obj provide a second set of candidates. Note that theirreliability is often lower, compared to camera motion-driven candidates.For example, high “action” values can occur when motion estimation failsand do not necessarily represent true object motion.

There are at least two ways of quantifying secondary motion. One can usethe final data values after the M-estimator to compute the deviationfrom the estimated global motion model, as taught by J. -M. Odobez andP. Bouthemy. Another way is to compensate each pair of frames for thecamera motion. Motion compensation is a way of describing the differencebetween consecutive frames in terms of where each section of the formerframe has moved to. The frame I at time t+dt is compensated for thecamera motion and object motion is given by Eq. 1.

The confidence function for object motion in a “fixed” segment is afunction of its length. A long period without camera motion indicatesparticular interest of the cameraman. First, the segment length l_(fix)(in reference

₁) is rescaled as a percentage of the total video duration such thatl_(fix)ε[0,100]. Moreover, it seems reasonable to assume that the gainin interest should be higher from a 1-second to a 2-second segment, thanbetween a 10- and a 12-second segment. In other words, the confidencefunction d_(fix)(obj) increases in a non-linear fashion. In a preferredembodiment of the present invention, this observation is modelled byx/(1+x). Therefore, $\begin{matrix}{{d_{fix}({obj})} = \frac{l_{fix} \cdot {obj}}{1 + \left( {l_{fix} \cdot {obj}} \right)}} & (13)\end{matrix}$

The confidence value for object motion in a “pan” segment is generallylower because the object motion is in the presence of large cameramotion. The confidence score is related to the translation amount duringthe pan: higher confidence is generally associated to objectmotion-based candidates during small translation. In a preferredembodiment of the present invention, a similar function is used withmodification: $\begin{matrix}{{d_{pan}({obj})} = \frac{10{\frac{a_{i}}{{th}_{pan}} \cdot {obj}}}{1 + \left( {10{\frac{a_{i}}{{th}_{pan}} \cdot {obj}}} \right)}} & (14)\end{matrix}$where the index i of the translation parameter a is either 1 or 2 (forthe horizontal and vertical axes).

The confidence value for object motion in a “zoom” segment is set tozero because object motion within a zoom segment is highly unreliable.Therefore, d_(zoom)(Obj)=0 and no candidate is extracted based on objectmotion.

Although the present invention is embodied primarily using camera motionand object motion cues, those skilled in the art can use complementarydescriptors, such as image quality (IQ), semantic analysis (e.g., skin,face, or expression) to improve the results at additional expense,without deviating from the scope of the present invention.

In the last step 50 of FIG. 1, final key frames 52 are selected from theinitial candidates 42. The confidence value of each candidate enablesrank ordering. To space out KF, at least one key fame (the highestranked candidate) is extracted per segment unless its confidence valueis too low. To fill in the user-specified number of key frames N_(KF),the remaining candidates with the highest confidence values are used. Iftwo candidates are too close in value, only the one with the higherconfidence value is retained. Preferred embodiments should useinformation from additional cues, including image quality (e.g.,sharpness, contrast) or semantic descriptors (e.g. facial expression) toselect the appropriate frame.

The present invention has been described with reference to a preferredembodiment. Changes can be made to the preferred embodiment withoutdeviating from the scope of the present invention. Such modifications tothe preferred embodiment do not significantly deviate from the scope ofthe present invention.

PARTS LIST

-   10 input digital video-   20 global motion estimation-   30 video segmentation-   31 video segments-   32 camera motion classes-   40 candidate frame extraction-   41 rules-   42 candidate frames-   50 key frame selection-   51 key frame number-   52 key frames

1. A method of analyzing a digital video clip to determine candidateframes for subsequent key frame selection, comprising: a. performing aglobal motion estimate on the video clip that indicates translation ofthe scene or camera, or scaling of the scene; b. forming a plurality ofvideo segments based on the global motion estimate and labeling eachsegment in accordance with a predetermined series of camera motionclasses; and c. extracting key frame candidates from the labeledsegments and computing a confidence score for each candidate by usingrules corresponding to each camera motion class and a rule correspondingto object motion.
 2. The method of claim 1 wherein the predeterminedcamera motion classes include pan (left or right, and tilt up or down),zoom (in or out), fast pan or fixed.
 3. The method of claim 2 whereinthe rules include a pan rule, a zoom rule, a fast pan rule and a fixedrule.
 4. The method of claim 3 wherein the pan rule includes extractinga plurality of frames to cover the space of environment while reducingthe spatial overlap among the frames from a pan segment.
 5. The methodof claim 3 wherein the pan rule includes extracting a frame located at apoint when the pan motion is slowed down.
 6. The method of claim 3wherein the zoom rule includes extracting a candidate frame at anendpoint of the zoom-in or zoom-out segment.
 7. The method of claim 3wherein the fast pan rule includes not extracting a candidate frame froma fast pan segment.
 8. The method of claim 3 wherein the fixed ruleincludes extracting a candidate frame located at a midpoint of the fixedsegment.
 9. The method of claim 1 wherein the object motion ruleincludes extracting a candidate frame for a fixed segment with aconfidence score related to the segment length, extracting a candidateframe for a pan segment with a confidence score related to a translationamount during the pan, and does not extract candidate frames for objectmotion based frames for fast pan and zoom segments.
 10. A method ofanalyzing a digital video clip to determine key frames, comprising: a.performing a global motion estimate on the video clip that indicatestranslation of the scene or camera, or scaling of the scene; b. forminga plurality of video segments based on the global motion estimate andlabeling each segment in accordance with a predetermined series ofclasses; c. extracting key frame candidates from the labeled segmentsand computing a confidence score for each candidate by using rulesassociated with each label; and d. selecting key frames from thecandidate frames based on the confidence score of each candidate. 11.The method of claim 10 further including ranking the selected key framesin accordance with the confidence score.
 12. The method of claim 11wherein step d includes employing the ranking and a user specifiednumber to select the key frames.
 13. The method of claim 12 whereinemploying the ranking and a user specified number to select the keyframes includes selecting at least one key frame from each segment ifthere are confidence scores above a pre-determined threshold.
 14. Themethod of claim 13 wherein employing the ranking and a user specifiednumber to select the key frames includes selecting key frames from theremaining candidates with the highest confidence values to fill the userspecified number of key frames.
 15. The method of claim 10 wherein thepredetermined camera motion classes include pan (left or right, and tiltup or down), zoom (in or out), fast pan or fixed.
 16. The method ofclaim 15 wherein the rules include a pan rule, a zoom rule, a fast panrule and a fixed rule.
 17. The method of claim 16 wherein the pan ruleincludes extracting a plurality of frames to cover the space ofenvironment while reducing the spatial overlap among the frames from apan segment.
 18. The method of claim 16 wherein the pan rule includesextracting a frame located at a point when the pan motion is sloweddown.
 19. The method of claim 16 wherein the zoom rule includesextracting a candidate frame at an endpoint of the zoom-in or zoom-outsegment.
 20. The method of claim 16 wherein the fast pan rule includesdoes not extract candidate frames from a fast pan segment.
 21. Themethod of claim 16 wherein the fixed rule includes extracting acandidate frame located at a midpoint of the fixed segment.
 22. Themethod of claim 10 wherein the object motion rule includes extracting acandidate frame for a fixed segment with a confidence score related tothe segment length, extracting a candidate frame for a pan segment witha confidence score related to a translation amount during the pan, anddoes not extract candidate frames for object motion based frames forfast pan and zoom segments.